1. Field of the Invention
The present invention relates to wireless digital communications systems. More particularly, the present invention relates to a code-division multiple access (CDMA) communication system employing a base station having remotely located RF power amplification equipment.
2. Description of the Prior Art
CDMA systems provide an efficient use of the limited bandwidth of the RF spectrum, thereby permitting a greater amount of information transmission with less signal distortion than communications systems using other techniques, such as time division multiple access and frequency division multiple access.
In a CDMA communication system, an information signal at the transmitter is mixed with a pseudorandom spreading code which spreads the information across the entire bandwidth employed by the system. The spread signal is upconverted to an RF signal for transmission. A receiver, identified by the pseudorandom spreading code, downconverts the transmitted spread-spectrum signal and mixes this with the pseudorandom spreading code to reproduce the original information signal.
A prior art CDMA communication system is shown in FIG. 1. The communication system has a plurality of base stations 201, 202, . . . , 20n connected together through land lines via a local public switched telephone network (PSTN) or by a wireless link. Each base station 201, 202, . . . , 20n communicates using spread spectrum CDMA transmissions with mobile and field subscriber units stations 221, 222, . . . , 22n located within its cellular area. Because a signal transmitted from one base station appears as noise to another base station, and thereby interferes with the ability of the second base station to receive transmissions from subscriber units located within its cellular area, it is desirable to carefully limit the amount of power transmitted from each base station 201, 202, . . . , 20n.
A CDMA base station typically has equipment such as an RF power amplifier and related electronics (not shown) located in a sheltered ground station for reception and transmission of RF signals and an antenna, or multiple antennae, mounted at some distance from the sheltered ground station. A significant amount of RF energy is lost through the cable connecting the ground station to the antenna. Accordingly, it is advantageous to co-locate or integrate the RF amplifier and related electronics with the antenna. This arrangement results in cost savings and energy efficiency because it permits the use of a lower power RF amplifier than is necessary when the RF amplifier is located at a distance from the antenna. Although RF power is still lost in the cable which connects the ground station with the RF amplifier located at the antenna, far less power is lost than in the case where the RF amplifier is located in the ground station.
In order to control the amount of power transmitted at the antenna, the power loss through the cable must be measured and compensated. In addition, the power loss through the cable varies with temperature. This variability causes a variation in signal level to the RF amplifier, which can result in overpowering or underpowering the RF amplifier. Overpowering the RF amplifier can cause interference with neighboring cell sites and can cause distortion of the transmitted signal which produces additional undesirable noise. Underpowering the RF amplifier can result in the transmission of a signal that is too weak to effectively communicate with the subscriber units within the cell area of the transmitter.
U.S. Pat. No. 5,634,191 discloses an arrangement for a TDMA system for compensating for cable loss between separate communication system units. The transmit power level of the communication is quantized at the first communication unit and sent as data together with the communication signal to the second communication unit. At the second communication unit, the power level of the communication is again measured and compared to the quantized power level. A signal attenuator adjusts the communication power level at the second communication unit to be equivalent to the quantized power level.
The measurement of the power of an RF signal received by a base station is also critical to an effective CDMA system. Since this measurement is typically made at the ground station, variability in power loss through the cable also adversely affects the accuracy of the received power measurement.
Accordingly, there is a need for continuous, automatic compensation of the power loss in the cable between the antenna and the ground station in order to effectively control RF transmission power from the base station and more accurately to measure power received from subscriber units.